The present invention relates to a liquid coupling system for video projectors and more particularly to a liquid system for coupling a CRT to a projection lens in a video projector in which enhanced brightness and contrast are obtained without accompanying increases in x-ray emissions.
There has developed substantial interest during the past few years in big screen projection of television images. In one common type of projection system, the images are projected on a screen from the face of a CRT through a lens system. In the case of color, three separate CRT's are employed, one for each primary color, and projection is through a lens system for each CRT and the pictures converge on the screen. Typically, the screen would be in a darkened room or theater as the amount of light available for projection is dependent on the brightness of the CRT's.
With the increasing popularity of projection television there has been accompanying interest in the use of large screen TV, or projection TV, in surroundings other than theaters where it is not possible, convenient, or desirable to darken the premises.
Examples of such surroundings are in homes and other non-theater locations during daylight hours or in reception areas of public places, such as hotel lobbies and the like.
Because of ambient lighting conditions in the surroundings described above, existing video projection systems tend to produce screen images which are not as bright as they should be and this condition tends to limit the growth in use of video projection systems.
Alternate types of systems are available, such as light/valve, but they usually are much more expensive.
As pointed out earlier, in present acceptable methods of video projection, the CRT is interfaced to a projection lens which projects the image. The brightness of the resulting image is limited by several factors, including screen reflectivity, CRT-to-lens coupling medium, lens light transmission characteristics, and the energy level applied to the CRT, that is, the brightness displayed on the television screen.
Screens in use today are highly reflective in nature and it does not appear that much improvement in brightness can be obtained by making further changes in screen technology. Similarly, it appears that the present use of good quality lens systems precludes improvements in this area as well.
Hence, the remaining two variables in the video projection system, namely, coupling medium and energy level applied to the CRT, have drawn the attention of workers in the field interested in increasing the brightness and improving the contrast displayed on the large screen.
It has been demonstrated that using a liquid such as ethylene glycol or water and glycerine to couple the CRT to the lens does improve brightness and contrast as well as provide a medium for heat dissipation. When the energy level of the CRT is increased, the level of brightnes on the face of the tube is increased which results in greater brightness on the large screen as well as additional heat to be dissipated.
Thus, substantial increases in the CRT energy level appear to be the most direct and effective means of improving in projection systems screen brightness to a point where the result is acceptable under the conditions described above.
Unfortunately, it has been discovered that these increases in CRT energy level generate more heat than can be accomodated by the aforementioned liquid couplings, and furthermore, produce unacceptable levels of x-ray emissions. Efforts to negate these effects include forced air cooling and the use of doped faceplates to absorb or attenuate the x-ray emissions.
The approach described above adds substantially to the cost and bulkiness of the video projections system as well as raising reliability problems over a long period of use due to possible failure of the forced air cooling system, build up of dust over a period of time causing static charges to develop, as well as other problems.
Efforts have been made to improve liquid coupling to avoid the use of forced air cooling but these have not been notably successful because existing systems use separately sealed lenses and liquid cells to avoid leaking. As a result, there is present an air gap which limits the improvement in brightness which can be obtained. Also, there is a tendency for dust to build up in air gaps causing attenuation of the light passing therethrough.